Investments - Summary of Financial Information (Detail) - Donegal Financial Services Corporation [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Balance sheets:
Total assets	$ 560,519		$ 560,519		$ 567,935
Total liabilities	478,652		478,652		487,604
Stockholders' equity	81,867		81,867		80,331
Total liabilities and stockholders' equity	560,519		560,519		$ 567,935
Income statements:
Net income	$ 1,634	$ 802	$ 2,945	$ 1,285